The correct option is
B suffer
Option B: 'Suffer' means 'to undergo or experience pain or distress'.
This word is relevant to the sentence: infants and preschool children experience distress from malnutrition and other ailments, as a result'.
Hence option B is correct.
Options A, C and D:
'Affect' means 'to produce an effect or change in'.
'Fear' means 'to be afraid of'.
'Save' means 'to rescue from danger or possible harm'.
These words do not complete the sentence in meaning.
'Save' is an active verb and does not express the experience that the infants undergo.
Hence these options are wrong.